A buck-boost converter converts an input voltage to an output voltage, and may work under buck mode, boost mode or buck-boost mode. Usually, the buck-boost converter works under buck mode when the input voltage is higher than the output voltage, works under boost mode when the input voltage is lower than the output voltage and works under buck-boost mode when the input voltage is close to the output voltage.
FIG. 1 schematically shows a prior art buck-boost converter 10. In the example of FIG. 1, duty cycles of the power switches PA˜PD are controlled by an error amplified signal Vcom. As a result, the ripple and noise of the error amplified signal Vcom will cause switching ripple of the output voltage Vout during mode transition and under buck-boost mode. Meanwhile, under light load, the efficiency of the buck-boost converter 10 is very low, and the mode transition is slow which resulted in a large output voltage ripple. Furthermore, the control circuit of the buck-boost converter 10 which is partly shown in FIG. 1 is very complicated.